Move all files from the docs repo to top-level (jfx) repo

In preparation for retiring the docs repo (see RT-18139) we need to move all of the files currently under the docs repo to the top-level repo.

The easiest way to do this is to create a doc-files directory in jfx and copy all of the README files from docs/** to jfx/doc-files/** keeping the same structure. Then we need to fix the top-level build file and the reference from rt-closed to use the new directory name. We can then restructure from there as appropriate.